A static mixer, which is sometimes called a motionless mixer, is a simple device with no moving parts. It consists of internal baffles or elements inside a plastic tube. However, this seemingly elementary device can effectively mix two flowable liquids. As adhesive components are forced through the mixer, they are repeatedly divided and recombined, creating a uniform mixture.
Static mixers are frequently chosen when users encounter too many difficulties with conventional adhesive handling methods—for example when components are scooped into a cup, handmixed and transferred to a dispensing container. Using a static mixer provides many benefits, including consistency of mix and elimination of the introduction of air from the mixture. The latter is an essential precaution. Air causes voids in cured bond lines that can lead to bonding failure. Overall, process control in hand mixing is difficult to maintain, and this can create serious bonding problems, as well as problems with waste, cost and safety.
Flow rate should be considered when choosing between handheld cartridges and MMD systems. During two-component adhesive operations, the components are kept separate in a preproportioned, molded plastic cartridge or in machined steel cylinders before being introduced into a static mixer. The user must calculate the amount of pressure necessary to keep the two liquids flowing at the appropriate rate for a specific application.
For example, high-viscosity components may need significant force to move the liquids at a correct flow rate. Understanding the maximum flow rate that a handheld cartridge system can generate provides a data point for deciding between a cartridge vs. MMD equipment.
MMD systems are automated and can dispense higher volumes of liquid than handheld cartridge systems. “Volume is one of the most common reasons for switching to MMD equipment; although volume alone is not a deciding factor. In many instances, end users upgrade from handheld cartridge systems to MMD equipment, particularly when time-controlled adhesive shots are needed. This is frequently the case with assembly line and robotic applications, which often are handled with MMD systems,” notes Don Leone, general manager at Ashby Cross Co. Inc., Newburyport, MA.
In contrast, airplane manufacturers use handheld cartridge systems because their bonding operations are spread over large areas. Also, when inside the airplane, assemblers must work in an extended area that has many tight locations. Plant maintenance personnel are likely to use handheld systems for similar reasons.
Complex manufacturing or maintenance operations often use many different adhesives in a wide range of applications. Changing adhesives in MMD systems is generally laborious. But many operators feel that changing adhesives in a handheld system is no more difficult than changing a cassette in a video cassette recorder. Handheld systems are chosen in many industrial settings for their simplicity. Even in a less complex operation, such as repairing cracks around swimming pools, an end user will find a handheld cartridge system beneficial because of the size and unique qualities of each area needing repair.
MMD systems are primarily used inside buildings and plants. Handheld systems are used both inside and outside. Handheld cartridge systems are manufactured in a limited number of volumetric ratio systems—1-to-1, 3-to-2, 2-to-1, 4-to-1 and 10-to-1. MMD systems are always customized, and this yields a nearly unlimited set of volumetric ratios. Outside of structural applications, many ratios are not compatible with the discrete ratios offered in cartridge systems. A ratio such as 100-to-36 or 100-to-28 requires MMD equipment.
Flow rate, volume, production and related requirements often dictate a straightforward choice between handheld cartridge and MMD systems. Nonetheless a handheld cartridge system is often an excellent choice for an adhesive application before moving to the more expensive MMD solution. An adhesive manufacturer or end user can experiment with handheld cartridge systems to test variables that affect static mixer performance for specific applications, such as temperature, humidity, length or diameter of the mixer, and curing time and work life of the adhesives.
Needing to achieve a consistent mix underscores the importance of experimenting with both manual and pneumatic dispensers, because the type of dispensing gun and its use can influence adhesive outcomes. Each person operating a manual dispensing gun may have a different dispensing style and hand strength.
Although manual guns are the most prevalent, a pneumatic gun may be needed for static mixer usage. Pneumatic guns can provide constant pressure on the cartridge and mixer, thereby minimizing problems caused by constant flexing. Problems arising from flexing can be mitigated to some degree by using plastic cartridges with thick walls, which are more rigid and stronger. This introduces another variable in the decision process for manufacturers and end users.
An end user can take an integrated approach and select dispensing tools and static mixers made specifically for the cartridge being dispensed. This can keep a company from using dispensers that are too powerful. In such situations, the dispenser’s mechanical advantage may be offset by flexing and subsequent mixing problems.
When using a handheld plastic cartridge, an important step is to experiment with a manual gun to measure the effects of flexing and the individual characteristics of the different employees applying the adhesives. Combining this approach with testing various static mixer configurations can provide a comprehensive testing model.
The following set of questions can be used to test various static mixers:
• Is the static mixer’s outlet orifice the appropriate size to deliver the correct amount of adhesive?
• What inside diameter and number of mixing elements create the right flow rate? Is the static mixer the right length for the application?
• Is the adhesive being dispensed in locations that are difficult to reach? Consider using an adhesive to glue a metal fastener inside a hole in concrete. The application may require that the adhesive bottom out at the base of the hole to avoid trapped air. A custom mixer with an extension would be an effective solution in this case.
• Does the application require specialized attachments, such as a luer lock adapter, ribbon spreader tip, bent ending or customized flexible tubing?
• How much “content volume” waste can be afforded?
• Does the static mixer require a shroud? Shrouds should always be used in case the operating pressure exceeds the static mixer’s pressure rating. A shroud is an aluminum device with internal threads that hold it in place over the static mixer. Shrouds prevent the catalyst or resin components from bypassing the mixer elements and streamlining down the inner wall, preventing optimal mixing.
• Are the pressures high enough to require stronger elements, such as substituting polyacetal for polypropylene elements, which means a higher cost? If there is too great a pressure drop, a static mixer may not keep its shape, and the components could pass along the mixer walls instead of being correctly mixed. Likewise, if the elements break, the system could pass along broken plastic fragments.
• Would a shroud help with equipment positioning? Or in an application that involves heating?
Ken Lambert, product manager at ITW Plexus (Danvers, MA), emphasizes the importance of correct selection. “We view the static mixer as a central part of the adhesive system. Choosing a device with the correct configuration should not be an afterthought. Our approach is to recommend static mixer combinations that have been successful in our laboratory testing and field experience. It is an important support we provide as an adhesive manufacturer,” he says.
Adhesive manufacturers often run tests that are more extensive and rigorous than end users would implement. An adhesives manufacturer might chart work life, gel and curing times for various adhesives. This data is then distributed to end users with disclaimers outlining chemical properties and operating limits based on specific applications of the adhesives, and the conditions under which they are dispensed. An end user then can take the specifications and test various dispensing systems under different operating conditions, such as varying temperature or humidity, or with different number of elements or special attachments.
Another useful experiment involves evaluating how the curing rates of adhesives interact with various static mixers. If there are pauses in the dispensing phase, over time any curing that occurs in the tube could alter the effective diameter of the mixer. Changes in the effective diameter can negatively impact the chemical and physical properties of the adhesive. Useful experiments must consider the consequences of the materials hardening in the tube.
If the application requires frequent replacement of the static mixer due to hardening, the end user may have to weigh the benefits of (a) running the mix and purging, vs. (b) stopping a dispensing process with some frequency and throwing away the mixer.
Finally, financial considerations can influence end users to adopt different testing approaches. Some end users, who will eventually use MMD equipment, report that initial testing, which incorporates off-the-shelf cartridges, is a more cost effective method as compared to testing with MMD equipment.
In short, there is no formula that provides consistent answers. Simple guidelines like “all epoxies need 24 elements” can be dangerous. An acceptable mixer configuration for one application may not be appropriate for another, even if the same adhesive is being dispensed.
Testing and experimenting is a reliable path to choosing the right static mixer for a two-component adhesive. It is the shared responsibility of the adhesive manufacturer and end user to identify the static mixer that consistently mixes the adhesive in such a way that when cured, the adhesive reliably meets peel strength, shear strength, tensile strength and other specifications. Both the static mixer manufacturer and the MMD equipment manufacturer play an important role. They actively assist adhesive manufacturers and end users in identifying the variables to consider and test, helping them choose the right static mixer for a specific two-component adhesive.
This article originally appeared in ASSEMBLY Magazine, May 2003.